The Permian and Mesozoic sections of Vilyuy basin, which lies west of Lena River in eastern East Siberia, consists largely of permeable or weakly permeable sand-silt rocks. Impermeable units are not persistent in thickness nor in clay components and are on the whole subordinate in the section. When one considers the large size of the structures present and the abundance of coal deposits that contain great amounts of organic matter, it can be supposed that the amount of hydrocarbons that are trapped is a function of the sealing capacity of the barrier units.
Among the regional gas-oil-water seals in Vilyuy basin are Nedzhelin and Monom Formations of the Lower Triassic and Suntar Formation of the Lower Jurassic. These regional seals control all the main gas pools in the Permo-Lower Triassic and Lower Jurassic productive complexes. In individual areas a thin member of clays and siltstone in the middle of the Tagandzhin Formation of the Lower Triassic has good sealing properties.
The Nedzhelin seal consists of the Nedzhelin Formation of the Lower Triassic Induan Stage. It controls pools in the Upper Permian and also in sandy-silty strata within the Nedzhelin itself. Total thickness of this formation is about 100 m, decreasing toward the margins of the basin.
Nedzhelin Formation is the seal for the main gas condensate pools of Khapchagay arch and the Srednetyung structure. In several areas overpressure in the Permo-Triassic complex appears to have led to migration of much gas, which formed pools in the overlying Lower Triassic complex.
The middle Tagandzhin seal is local. Only in the central part of Linden depression does its thickness exceed 30 m, and only in two zones does thickness of swelling clays reach 10-20 m. In these zones this seal is responsible for gas pools in the Lower Triassic.
The Monom seal is the most important barrier in the Mesozoic section of Vilyuy basin. It controls the main gas condensate pools. Maximum thickness of this seal is in the cantral and northwest parts of the region where it reaches 220 m. Elsewhere it is 100-150 m thick. In the south on Khapchagay arch the thickness of the swelling clays is only 10-40 m; this is a "hydrogeologic window" where the seal is not effective.
The Suntar seal consists of clays that contain varying amounts of siltstone and limestone. This unit is very widespreadf in Vilyuy basin and is generally 60 m thick. Thickness of swelling clays is 20-35 m. Gas pools are associated with this seal only in parts of the Khapchagay arch where the underlying Monom seal is not effective.
In the study area the height of the gas column is related to thickness of swelling clays.
Taken from V. Ye. Bakin, 1979; digested in Petroleum Geology,
vol. 19, no. 10, p. 504-508, three isopach maps of seals.
Copyright 2002 James Clarke. You are encouraged to print out this
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